Model membranes composed of two types of long chain phospholipids, one unsaturated and one saturated, along with cholesterol can exhibit two coexisting fluid phases (liquid disordered (Id) and liquid ordered (l(o))) at various temperatures and compositions. Here we used 1D and 2D H-2 NMR to compare the behavior of multilamellar dispersions, magnetically oriented bicelles, and mechanically aligned bilayers on glass plates, all of which contain the same proportions of dipalmitoleoylphos-phatidylcholine (DPoPC), dimyristoylphosphatidylcholine (DMPC), and cholesterol. We found that multilamellar dispersions and bilayers aligned on glass plates behave very similarly. These samples were close to a critical composition and exhibit exchange of the lipids between the two fluid phases at temperatures near the Id to l(d)-I-o phase boundary. On the other hand, when a short chain lipid is added to the ternary long chain lipid/cholesterol mixture to form bicelles, the phase behavior is changed significantly and the l(o) phase occurs at a higher than expected temperature. In addition, there was no evidence of exchange of lipids between the I-d and l(o) phases or critical fluctuations at the temperature where the bulk of the sample enters the two-phase region for these bicelles. It appears that the addition of the short chain lipid results in these samples no longer being near a critical composition.